This invention concerns a radiation-curable composition. In particular, this invention concerns a radiation-curable composition that can be used as a base coat for, for example, electrostatic offset inks.
In electrostatic offset printing an image is formed by directing one or more pigmented inks or toners on to a selectively charged drum, which are then transferred, either directly or via an intermediate member, on to a substrate. Electrostatic offset printers are made and sold by, for example, Indigo NV and by Xeikon SA.
Inks used in offset printing have difficulty adhering to certain substrates such as plastics. This can be overcome by modifying the surface of the substrate. The surface can be modified by the application of a preliminary base coat comprising, for example, a solvent-based resin such as, for example, a polyamide, a butadiene resin or an ethyleneacrylic acid resin (see JP 10076744). When a solvent-based resin is used, the solvent needs to be evaporated before printing, which can be slow, hazardous and offensive, and is environmentally unfriendly.
WO 98/49604 discloses a copolymer receptor layer which is applied by extrusion to a polyvinyl chloride backing to improve abrasion resistance of a printed image. The copolymer receptor layer can be used to improve print quality on a polycarbonate backing layer. In both of these applications, we are told that the copolymer receptor layer adheres to the polyvinyl chloride backing layer in the absence of UV radiation.
The aim of the present invention is to provide a base coat that can be used on difficult substrates, such as plastic substrates, that are printed using, for example, offset printing.
A further aim of the present invention is to provide a base coat that does not contain volatile organic solvents.
A further aim of the present invention is to provide a base coat that can be applied by screen process printing or by any other suitable coating or printing process.
In accordance with the present invention there is provided a radiation-curable composition for use as a primer base coat for electrostatic offset printing, the composition comprising:
(i) an ethylenically unsaturated monomer, oligomer or polymer that is capable of being photopolymerized;
(ii) a photoinitiator that is capable of initiating the polymerization of the ethylenically unsaturated monomer (i) when irradiated with ultra-violet light;
(iii) optionally, a pigment;
(iv) an inorganic transfer agent; and
(v) an adhesion promoter.
In accordance with the present invention there is also provided a method for printing a substrate, the method comprising a stop of applying a base coat to the substrate before printing, the base coat comprising the composition defined above.
In accordance with the present invention there is also provided use of the composition defined above as a base coat for printing.
The base coat may be applied to a substrate using, for example, silk or screen process printing, flexographic printing, curtain coating, roller coating and spin coating.
In use, the base coat is applied to a substrate and then irradiated using ultra-violet light. The cured base coat possess excellent properties for the reception of images produced by electrostatic offset printing. The base coat can be used, for example, for printing on to compact discs, labels or similar plastics articles.
The monomer, oligomer or prepolymer (I) is preferably an ester of acrylic or methaorylic acid, such as, for example, phenoxyethyl acrylate, hexanediol diactylate, tri(propylene glycol) diacrylate, trimethylolpropane triacrylate, polyethylene glycol diacrylate hydroxyathyl methaerylate, trirnethylolpropane triacrylate, tetrahydrofurfuryl acrylate, lauryl acrylate, octyl acrylate, decyl acrylate, acrylates of polyurethane, polyester, polyether, melamine or epoxy resins, ethoxylated or propoxylated derivatives of any of the aforementioned acylates or methacrylates; or a N-vinylamide such as, for example, N-vinylcaprolaotam or N-vinylformamide; or a mixture thereof. The monomer, oligomer or prepolymer is preferably present in an amount from 10 to 80%, more preferably 40-60%, by weight of the composition.
The photoinitiator (ii) is of the typo known to produce free radicals when irradiated. The photoinitiator is, for example, benzophenone. 1-hydroxycyclohexyl phenyl ketone, 2-benzyl-2-dimethylamino-(4-morpholinaphenyl)butan-1-one, benzil dimethylketal, isopropyltnoxanthone, bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or a mixture thereof.
Colourless or nearly colourless materials are preferred; however, yellow coloured photoinitiators may also be used. The photoinitator is preferably present in an amount from 0.5 to 20%, more preferably from 2.5 to 8%, by weight of the composition.
The inorganic transfer agent is required to ensure good transfer of electroink from the electrostatic offset press blanket to the cured basecoat. The inorganic transfer agent (iv) is preferably a silica or a modified silica. Suitable silicas are Aerosil 200 (available from Degussa) and Cab-o-sil TS610 (available from Cabot), or a mixture thereof. It is particularly advantageous to use commercial materials in which the silica or modified silica is dispersed in an ethylenically unsaturated monomer ingredient of type (i), such as, for example. Highlink OG (available from Clariant). We have found that these transfer agents, especially silicas, render the final cured ease coat particularly receptive to electrostatic offset ink. They also have the advantage of giving a desirable xe2x80x98structurexe2x80x99 to the compositions, i.e. resistance to flow under gravity without excessive viscosity at higher shear. The inorganic transfer agent (iv) is preferably present in an amount from 0.5 to 50%, more preferably from 4 to 35%, by weight of the composition.
The adhesion promoter (v) is present to aid adhesion of the electrostation offset ink to the cured base coat. Suitable materials are polar organic nitrogen compounds having a molecular weight preferably greater than 200 and preferably containing two or more nitrogen atoms such as, for example, the amines sold by Huntsman Incorporated under the name Jeffamine D230 (a diamine); and Jeffamine T403 (a triamine); or polyamides, such as, for example, Solsperse 32000 (available from Zenoca), described as a polymer acid salt/polymer amide; or a mixture thereof. It is known in the art that amines may be used to act as synergists for photoinitiators; however, in this invention, the amines are added to aid the adhesion of the polar electrostatic offset ink to the base coat. The adhesion promoter is preferably present in an amount from 5 to 25%, more preferably from 11 to 19%, by weight of the composition.
Optionally, the following additives may also be present: passive resins (i.e. resins that are not cured by ultra-violet light) to stabilise the viscosity of the composition and improve the physical properties of the cured base coat; dyes or pigments, especially white pigment to form an opaque background for the electrostatic offset print; stabilisers to prevent deterioration of the composition to heat or on storage; flow aids; biocides; defoamers; and surfactants.
The components of the base coat are preferably selected so that they are compatible with each other and provide a mixture stable on storage, with the required rheological and other properties. In particular, the compositions can preferably be formulated to give a rheological profile suitable for application by screen-printing. For this purpose their gel strength, as measured as the viscosity at 25xc2x0 C. on a Brookfield RVDT digital viscomoter at Sr.p.m using a No.6 spindle, preferably lies between 18 and 1500 poise, more preferably between 100 and 600 poise, and their viscosity under higher shear, as measured on the Rotothinner (supplied by Sheen Instruments), is preferably between 3 and 65 poise, more preferably 20 and 50 poise. Alternatively, the compositions can be stared and supplied in two parts: the adhesion promoter being in one part and the rest of the composition being in the other part.
The substrate may be polycarbonate, which is used in the production of compact discs or other plastics materials such as, for example, poly(vlnyl chloride), toils or papers. Preferred substrates are plastics articles such as labels and overlay film, especially uncoated and spin-coated polycarbonate compact discs or DVD""s.
The substrate may be printed using a screen stencil or by any other appropriate printing method, and then hardened by exposure to ultra-violet light or cold-cure systems. Typical exposures are at a speed of 27-50 meters/minute on a Svecia u.v. dryer having two medium pressure mercury lamps operating at 80 watts/centimetre, or at up to 56 impressions per minute with two lamps at 100% power on a Kammann K15 printer. After curing, the substrate bearing the base coat is especially suitable for receiving electrostatic offset ink.
The invention will now be described, by way of example, with reference to the following Examples (all pans are by weight):